Kopp’s rule: For which type of substances is Kopp’s rule primarily used to estimate heat capacity?

Introduction / Context:
Kopp’s rule is a classical empirical guideline in thermodynamics used to estimate the molar heat capacity of crystalline solids from the sum of atomic contributions. It is often referenced in materials and inorganic chemistry when data are scarce.

Given Data / Assumptions:

• Kopp’s rule applies to solids, especially inorganic crystalline compounds.
• It approximates molar heat capacity at constant pressure near room temperature.
• Gas-phase heat capacities typically require different models (e.g., equipartition, statistical mechanics, correlations).

Concept / Approach:
The rule states: the molar heat capacity of a solid compound is approximately the sum of the atomic heat capacities of its constituent elements multiplied by their stoichiometric counts. It works reasonably for many ionic solids and simple crystals.

Step-by-Step Solution:

Verification / Alternative check:
Compare to experimental Cp values. Deviations occur for complex structures or strong anharmonic effects, but order-of-magnitude accuracy is common.

Why Other Options Are Wrong:

• Gases (options b and c): Gas heat capacities vary strongly with temperature and molecular structure; Kopp’s rule is not intended for gases.
• Activation energy: A kinetic parameter unrelated to heat capacity.

Common Pitfalls:
Applying Kopp’s rule at very high or very low temperatures where solid heat capacity deviates significantly from room-temperature trends.

Final Answer:
heat capacities of solids